There are certain apparatuses in use today which require temporary connection to service vehicles. For example, many vehicles require temporary connection to a fuel re-supply truck, and some fixed structures require either that or temporary connection to a battery recharging vehicle. Most large aircraft require temporary connection to ground power units (GPUs) which are typically mounted on a trailer towed by a “tug” and which supply electric power to an aircraft before its engines are started. Many turbine-powered aircraft further require temporary connection to an “airstart” unit to spin the engines prior to ignition. A problem with such temporary attachments is that there exists the potential for the service vehicle inadvertently to be driven away from the apparatus being serviced before detachment from the apparatus being serviced, almost certainly causing damage to the apparatus and/or the vehicle, and in addition possibly injuring personnel. Many incidents have been logged of aircraft GPUs or airstart trailers being driven off while the CPU umbilical is still attached to aircraft, causing, at a minimum, damage to the receptacle on the aircraft. Not only is such damage costly to repair, but also admits the possibility of exacerbated or even catastrophic damage to the aircraft in flight.
The system currently in use to prevent such incidents; involving CPU connections to aircraft consists simply of detaching the CPU towbar from the tug while the CPU is attached to the aircraft, raising the towbar to a vertical position, and placing a safety-yellow “sock” over it to signify to personnel that the CPU is attached to the aircraft. Procedures require that the sock not be removed from the towbar until the CPU is disconnected from the aircraft. The weakness in this procedure is that if someone should fail to follow it, the towbar can, and therefore inevitably sometimes will, be reattached to the tug and the tug driven away without the umbilical having been first detached from the aircraft.
It would be possible to arrange a safety interlock that would interconnect the ignition switch on the tug with the umbilical plug and receptacle combination on the aircraft. In other words, a sensor, such as a switch embedded in the CPU plug, could be electrically arranged from the plug to the CPU and thence from the GPU towbar to the tug, to keep the tug starter circuit open as long as the plug was in the receptacle. The main drawback here is expense of making, installing, and maintaining the interlock system itself on the GPUs and the tugs. Also, if it is relied upon, a failure in it would practically guarantee an occasional damage incident. As in all critical service routines where human life is at stake, it is important to assure that, as a final step in preparation for aircraft flight, human beings are alerted to hazards and allowed to take corrective action. This is why the preflight walkaround has been the capstone of aviation safety since the beginning of flight. An uncomplicated, and therefore inexpensive and reliable, system to warn personnel of an unsafe condition involving service vehicles is needed.